1) Field of the Invention
The present invention relates to a copper-based oxidation catalyst having a stable and catalytically highly active surface.
2) Related Art
It has been well known that the copper surface is active on oxidation reaction of formaldehyde, etc. and the copper surface has been applied also in catalysts for electroless copper plating, etc.
When electroless plating is applied to a non-conductive (dielectric) substrate, it is necessary to deposit a plating catalyst on the non-conductive substrate in advance. Palladium is known as such a catalyst and is practically widely used. According to one procedure, palladium can be deposited on a non-conductive substrate by dipping a non-conductive substrate into an aqueous stannous chloride solution acidified with hydrochloric acid, and then dipping the substrate into an aqueous palladium chloride solution acidified with hydrochloric acid, thereby carrying out a redox reaction on the surface of the substrate according to the following process: EQU Pd.sup.++ +Sn.sup.++ .fwdarw.Pd+Sn.sup.++++
According to another procedure, a palladium colloid coated with stannous ions is used as a plating catalyst.
In the foregoing procedures using a palladium catalyst, palladium metal insoluble in an electroless plating solution may be released from the catalyst-deposited substrate and entered into an electroless plating solution, thereby giving rise to autolysis of the electroless plating solution itself. Furthermore, particularly the procedure using both of the aqueous stannous chloride solution acidified with hydrochloric acid and the aqueous palladium chloride solution acidified with hydrochloric acid has a risk of attacking the substrate, because the solutions are highly acidic. Still furthermore, the palladium catalyst belongs to a noble catalyst species, which makes the catalyst cost higher.
A copper colloid is known as another electroless plating catalyst besides the palladium. In the production of a printed circuit board using a copper colloid, for example, circuit formation on a non-conductive substrate by electroless plating, a plating resist is formed on non-circuit-destined parts, a copper colloid is deposited on circuit-destined parts and also on the plating resist, and then the copper colloid catalyst on the plating resist is removed by mechanical polishing (JP-A-62-271491). In that case, the copper colloid catalyst-deposited non-conductive substrate is dried by heating at 100.degree.-160.degree. C. to enhance the adhesion between the surface of the non-conductive substrate and the copper colloid catalyst. The dried copper catalyst is in an oxidized state and has no catalytic activity, and thus is subjected to a reduction treatment by a reducing agent. In the electroless plating using such a copper colloid catalyst, it takes much time in starting of electroless plating reaction. Alternatively, an active electroless plating solution, that is, an unstable electroless plating solution, must be used, as disclosed in JP-A 62-297472, page 6, line 19-page 7, line 2.
An aqueous solution of copper colloid catalyst can be prepared by adding dimethylamineborane to an aqueous solution containing copper ions, gelatin and polyethylene glycol at a pH of 1 to 2, thereby reducing the copper ions to metallic copper, and then adjusting the aqueous solution to a pH of 2 to 4, as disclosed, for example, in JP-A 61-23762.
Furthermore, a combination of copper and nickel is disclosed in JP-A 2-207844 as a catalyst for electrolytic reduction of carbon dioxide, etc. The catalyst is a reduction catalyst for electrolytically reducing a reducible compound such as carbon dioxide or carbon monoxide under a reduction potential substantially equal to the theoretical potential, thereby forming useful compounds such as methane, ethylene, etc.
Copper surface is very susceptible to oxidation, and, once oxidized, has no catalytic activity. Even if reduced, the resulting copper surface has a considerably poor activity, as compared with noble metal catalysts such as platinum, palladium, etc.
Nickel, on the other hand, has a high corrosion resistance and no activity on the oxidation reaction of formaldehyde, etc.
In the above-mentioned prior art procedure using a copper colloid catalyst, the copper colloid deposited on a non-conductive substrate is readily oxidized by exposure to the atmospheric air and deactivated. Deactivation of the copper colloid catalyst is considerable particularly when heated to 100.degree. to 160.degree. C., and complete reactivation is hard to obtain even by using a reducing agent. Metallic copper itself has a poor catalytic activity, and, when used as a plating catalyst, parts where no plating reaction takes place, that is, the so called plating non-deposited parts are highly liable to appear. Thus, the copper colloid catalyst has not been so far widely utilized.
It is also known that the metallic copper has an activity on the oxidation reaction of lower alcohols as a fuel for a fuel cell, such as methanol, etc. However, its activity is considerably lower than that of noble metals such as platinum, palladium, etc., and thus the metallic copper has not been so far utilized as an electrode material for a fuel cell.